This disclosure relates to a data storage media, and especially relates to a highly colored data storage media and methods for making same.
Digital Versatile Disks (or DVDs) provide significantly more storage capacity than CDs thus allowing the recording of up to 8 hours of video/audio content with high digital quality. For this reason DVDs are steadily growing in market volume. The number of DVDs sold on the market is expected to reach about 4 billion by 2004.
To achieve the high storage density in a disk that has the same overall dimensions as a CD, the disk construction has been changed. The most fundamental and visible difference is that a DVD is no longer made of a single 1.2 mm thick plastic substrate but of two 0.6 mm halves that are bonded together (“DVD Demystified:” by Jim Taylor, ed. McGraw Hill, 1998). In addition, the reading laser wavelength has changed from 780 nanometers (nm) in CDs, which is located in the near infrared part of the spectrum, to 635 or 650 nm for DVDs that is located in the visible spectrum. Depending on the type of DVD format (DVD-5, DVD-9, DVD-10, DVD-14, DVD-18, DVD-R, DVD-RW, etc.), the construction can vary substantially, going from a single side/single layer (DVD-5) to double side/double layer (DVD-18), not to mention the recordable formats such as DVD-R and DVD-RW. The tight specifications, e.g., imposed by the Optical Media consortium (European Association for Standardizing Information and Communication Systems (“ECMA”) Specifications #267 for 1.20 mm DVD-Read only disk), make DVD manufacturing a challenge. One specification requires that all DVDs show a PISUM8 test value (measurement of reading errors) of less than 280 even after temperature and humidity environmental aging. However, most optical disc manufacturers have even more stringent requirements and do not tolerate initial PISUM8 values of more than about 140 with an average value of less than 50.
The optical properties of the substrate are not the only requirements for DVDs. For example, the size of a data pit is much smaller in a DVD than a CD, which means that a DVD is more sensitive to resin quality (impurities) than a CD. Moreover, due to their thinner gage (0.6 mm instead of 1.2 mm), DVDs are also more challenging to mold than CDs. Therefore, the rheology (flow characteristics) of the resin used to make DVDs is important and must be tightly controlled. Additionally, inconsistencies in the thickness of the disk body are also a source of defects in optical disks. A DVD is much more sensitive to the presence of such defects because it is not only thinner and read in the visible spectrum, but it is also read at a higher rotational/scanning speed than a regular CD. The most advanced multi-layered DVD formats (i.e., DVD-9, DVD-14 and DVD-18) represent the highest degree of manufacturing difficulty because of the presence of both a fully reflective and a semi-reflective layer associated with 2 layers of pits. Single layered DVD formats (i.e., DVD-5 and DVD-10), do not have the semi-reflective layer and contain only one layer of pits. Because of the differences in the disk construction, reflectivity requirements are very different between single layered and multi-layered disks.
The challenges of making high quality DVDs become greater still when colored resins are employed in the substrate layers because of the varying quality of the available colorants. For this reason, DVDs have not generally been made with colored resins. Recently a new DVD product has been introduced to the market under the trademark EZ-D™. This DVD employs special colorants to limit the number of playable hours of a rented film in a DVD format. Several days after removal from its packaging, disk color changes from red to black preventing reading of the data by the laser, and the disk is rendered unplayable. With the exception of this specialized use of colorants, colored DVDs have not been generally available to the market except when the color effect was achieved by metallization (i.e., the data layer). For example a gold disk is obtained when gold is used instead of the typical aluminum reflective layer, and similarly, a bright silver look is obtained with silver or a silver alloy. Also, DVDs have been available in single-sided formats with colorants in substrates that were not intended to be read through during use of the DVD. Colored CD's are also known and commercially available for use with Sony Playstation I and II game consoles. These CD's have had a maximum dye loading in the read side substrate of 0.13 weight percent.
DVDs presently found on the market are largely made using clear (colorless) resins. These disks show good optical/electrical properties (reflectivity, jitter, parity inner (PI) errors, birefringence, etc.) as well as good mechanical properties (radial/tangential/vertical tilt), even after aging at 80° C. at 85% relative humidity (RH) for 4 days. The absence of highly colored DVDs in the marketplace, in spite of high customer interest to have such aesthetically pleasing products, has been the result of having no reliable means to ensure that the highly colored DVDs produced would consistently meet the optical and electrical properties and aging characteristics required by the optical media industry.
In general, to achieve an aesthetically pleasing appearance in the DVDs, colorant loadings of greater than 0.15 weight % relative to the weight of the resin are needed depending on the type of colorant and the appearance desired. But colorants vary widely in their impurity contents, especially in their metal impurity contents. At high colorant loadings these metal impurities can adversely affect the short-term properties and long-term aging characteristics of the highly colored DVDs.
The metal impurities present in colorants and highly colored resin formulations useful for DVDs fall into two categories: those which are larger than 10 microns, and those that are less than 10 microns or are soluble in the highly colored resin formulation. The removal of the metal impurities that are greater than 10 microns can be achieved by those skilled in the art using a series of melt filters with a screen size of around 10 microns during the melt extrusion of the resin. The removal of metal present which are less than 10 microns, or that are soluble in the highly colored resin formulation is much more difficult because these particles pass through conventional extrusion filters during the extrusion process. Very fine mesh size, specialty extrusion filters can remove more of the very small metal particles but their use can create severe production rate penalties, which can make the cost of producing the highly colored resin formulations prohibitively expensive. Also, if the metal impurities are soluble in the highly colored resin formulation, they cannot be removed by any filtration method. Another solution available to a resin manufacturer who wants to produce a highly colored resin for use in DVDs is to employ highly purified dyes such as those used in electronic applications. Such dyes possess 10 ppm or less metal impurities. But this solution adds prohibitively high costs to the highly colored resin formulations intended for use in DVDs. Moreover, electronic grade dyes are only available in limited colors.